This eye includes, inter alia, a cornea and a lens, separated by an anterior space. Each of these components provides optical refraction for proper vision. Defects in vision may result from a misshaped cornea which fails to provide the necessary refraction. For example, myopia (nearsightedness) occurs when the visual images come into focus in front of the retina of the eye, and this results in distant objects being viewed as out of focus. Hyperopia (farsightedness) occurs when the focal point of visual images is behind the retina of the eye, resulting in difficulty in viewing nearby objects. Astigmastism is an optical defect which occurs when an optical component such as the cornea or lens is irregularly shaped and the rays of the visual image fail to meet in a focal point, resulting in a blurred and imperfect image.
These defects have been treated in a variety of ways. Eyeglasses or contact lenses have been used, but may be expensive or inconvenient, or require changes as the visual acuity of the eye changes over time, and are practical only for humans and not in veterinary applications.
In more recent developments, myopia and astigmatism have been treated by a surgical procedure known as radial keratotomy. This procedure involves making radical cuts in the cornea from a central optical zone to change its shape. Many patients have reported satisfactory results, but the procedure not only requires great skill by the opthamologist but also weakens the cornea. It is also limited to about 8 diopters of refractive correction, and the patient may experience some reflection from the incisions resulting in degradation of night vision.
Another surgical solution which has been advanced uses eximer lasers to sculpt the cornea. By removing selected portions of the cornea through vaporization, the surgeon may correct the patient's vision. This involves scraping the epithelium (the thin outer layer over the cornea) away and then the eximer laser ablates the cornea. Unfortunately, this procedure is painful to the patient because of damage to the epithelium and Bowman's membrane, and requires a great deal of surgical skill, as well as involving delicate and expensive equipment.
As a result of certain drawbacks in other procedures, a technique enjoying increased popularity is lamellar surgery, which involves making lamellar (disk) cuts in the eye, and then removing and reshaping the disc before reattachment. The cut disk was frozen, reshaped and replaced in a procedure first developed by Dr. Barraquer known as keratomileusis. Later developments have refined this procedure and have attempted to overcome deficiencies in the technique. One recognized improvement has been the removal and retention of the uppermost layer of the cornea. This uppermost layer includes the epithelium and the Bowman's layer or membrane, which should be replaced for optimal healing of the eye. Removal of the epithelium and Bowman's membrane exposes the stroma of the cornea. The microkeratome removes a disc from the stroma of the desired thickness, and the surgeon then replaces the epithelium and Bowman's membrane like a cap over the lamellar cut. The preservation of the Bowman's membrane and epithelium allows the epithelial cells to reattach the cap over the stromal bed. More recent techniques, known as laser assisted in-situ keratomileusis (LASIK) uses a combination of the lamellar technique and eximer laser technique whereby after removal of the cap, the eximer laser ablates the stromal bed to provide the desired refractive correction, then the cap is replaced as described above.
A number of different keratomes have heretofore been developed, as shown, for example, in U.S. Pat. No. 5,591,174 to Clark et al.; U.S. Pat. No. 5,496,339 to Koepnick; U.S. Pat. No. 5,288,292 to Giraud, et al.; U.S. Pat. No. 5,133,726 to Ruiz et al.; U.S. Pat. No. 4,884,570 to Krumeich et al; and U.S. Pat. No. 4,662,370 to Hoffman et al.; and also in published International Application No. WO 93/06783. These keratomes involve a blade which oscillates from side to side to cut through the cornea as the blade moves longitudinally. The blade is typically driven by a cam and slot arrangement, the cam being connected to a motor drive. The blade is located beneath an applanator surface which flattens the arcuate surface of the eye for performing the cut, and also increases the interocular pressure within the eye, which is critical to a successful cut of the cornea. The depth of the disc cut is very precise, measured in microns, to provide proper optical correction.
Several problems are presented by such prior art microkeratomes. One problem is the weight of the instrument which is placed on the eye. The eye is coated with a lubricating liquid (tears) and the greater the weight borne by the eye, the greater the likelihood the microkeratome will slip.
Slippage of the microkeratome during surgery may have catastrophic results. Current microkeratomes place 5 or 6 ounces (about 150 grams) of weight offset from the eye.
Another problem associated with the current microkeratomes is that fluid may build up beneath the applanator adversely affecting the precision of the cut. The applanator may slide along the cornea during surgery. The sliding applanator should produce a film by hydrodynamic wedge action and when stationary should produce a squeeze film when pressed against the cornea. The possibility exists for a liquid to be entrapped between the blade and the applanator when the liquid film collapses at its edges by leakage. The presence of any liquid film reduces the thickness of the cut by occupying the space between the blade and the applanator. Such entrapment would reduce the thickness the most at the center of the intended cut, and if thicker than the intended thickness of the disc to be cut, then a central region of the cornea will not be cut, resulting in a "button-hole".
Another problem associated with button-holing is the lack of sufficient interocular pressure (IOP).
Another problem is that in prior art devices it is difficult to see the progress of the cut as it is being made. In certain situations this is a severe disadvantage as there is no way to stop the cutting process before damage is done.